1. Field of the Invention
The present invention relates to a method of transmitting a farewell message on a UPnP network, and more particularly, to a method and apparatus for transmitting a Byebye message when an operation of a controlled device in an UPnP network is abnormally terminated.
2. Description of Related Art
A universal plug and play (UPNP) network includes a controlled device, that is, a home network device, which is connected to an IP-based home network and a control point which controls the controlled device. The control point discovers the controlled device in the network through a discovery procedure defined in the UPnP standard. At this time, the control point may use an Simple Service Discovery Protocol (SSDP) notify message or an M-Search message. These messages will be described in detail below.
FIG. 1 is a view for explaining a method used by a controlled device 110 to discover a controlled device by using an SSDP notify message 141. The controlled device 110 transmits the SSDP notify message 141 to control points 120 and 130 through a multicast channel 140 in order to notify them that it is connected to a network. The SSDP notify message 141 includes information LOCATION on a location of a description file of the controlled device 110, Notification Type NT, information SERVER on a server, and a UUID value USN. The controlled device 110 can notify the control points 120 and 130 of an effective time period of accessing the network by using a value CACHE-CONTROL. More specifically, when the value CACHE-CONTROL is set to 1,800 seconds, the control points 120 and 130 consider the controlled device 110 to be normally connected to the network for 1,800 seconds, although the controlled device 110 does not transmit the SSDP notify message 141 again.
FIGS. 2A and 2B are views for explaining a method used by a control point 120 to discover a controlled device by using an M-Search message 151.
When the control points 120 and 130 transmit the M-Search message 151 through the multicast channel 150 in order to discover the controlled device 110, the controlled device 110 transmits a response message to the control points 120 and 130 through a unicast channel 160. Each of the control points 120 and 130 receiving the response message adds the controlled device 110 to a list of controlled devices which each of the control points 120 and 130 manages. These controlled devices can be identified by USN values included in the response message 161. Similar to the SSDP notify message 141, the response message includes the CACHE-CONTROL value, which the effective time period of the controlled device 110 is set to. More specifically, the controlled device 110 must transmit the SSDP notify message 141 indicating that the controlled device 110 is connected to the network before the CACHE-CONTROL time period elapses. When the controlled device 110 does not transmit the SSDP notify message 141, the control points 120 and 130 consider that the controlled device 110 is not connected to the network.
In addition, as shown in FIG. 3, when the controlled device 110 transmits a Byebye message through the multicast channel 170 at the time of terminating an operation, the control points 120 and 130 consider the controlled device 110 to be disconnected from the network.
Further, according to a UPnP discovery method defined in the conventional UPnP standard, the control points 120 and 130 cannot detect an abnormal termination of an operation of the controlled device 110. The abnormal termination of an operation of the controlled device 110 denotes a termination of an operation without transmitting a Byebye message 171. For example, the abnormal termination of an operation may occur due to crashing of a system of the controlled device 110 or unplugging of a network cable.
However, in this case, the control points 120 and 130 may consider the controlled device 110 to be a valid device for the CACHE-CONTROL time period. In other words, although the controlled device 110 disappears from the network, an application of the control point 120 or 130 is notified that the controlled device 110 operates normally. Due to such an incorrect notification, the application of the control point 120 or 130 may try to control the controlled device 110 which has already disappeared from the network. Particularly, in a specification of the digital living network alliance (DLNA) for setting up a home AV system using an UPnP AV technique, a recommended CACHE-CONTROL time period is 1,800 seconds. Therefore, most associated companies should comply with this recommendation. In this case, the control points 120 and 130 cannot detect the termination of an operation of the controlled device 110 for a time up to 1,800 seconds.
In addition, when the control points 120 and 130 periodically transmit the M-Search messages 151 to the controlled device 110 to detect the abnormal termination of an operation of the controlled device 120, the load on the network increases. On the other hand, if a new message is defined in the standard so as to detect the abnormal termination of an operation of the controlled device, the message may not be compatible with existing devices complying with the existing UPnP standard.